Floating rain gutter/ceiling tiles after a building is burned down

[Maarifrah]

Describe the bug

When burning down a building, sometimes ceiling tiles that have rain gutters on them don't collapse, even if they have no support. Observed behavior is that ceiling tiles without rain gutters on them consistently collapse without support, but it's not consistent if the ceiling tile has rain gutters.

Attach save file

test-trimmed.tar.gz

Steps to reproduce

(In the save provided, the building to the right has already been set on fire and you only need to wait).

1. Set a wooden building on fire. Try to choose one that will fully burn down on all sides, leaving no standing structural elements that can support a ceiling.
2. Wait until the fire burns out.
3. Check the remaining ceiling tiles to see which ones are left floating in the air without any support.

Expected behavior

No remaining ceiling/rain gutter tiles after a building has completely burned down as there is nothing that can support them.

Screenshots

Versions and configuration

• OS: Windows
  • OS Version: 10.0.19045.5011 (22H2)
• Game Version: 5d32050 [64-bit]
• Graphics Version: Tiles
• Game Language: System language []
• Mods loaded: [ Dark Days Ahead [dda], Disable NPC Needs [no_npc_food], Portal Storms Ignore NPCs [personal_portal_storms], Slowdown Fungal Growth [no_fungal_growth] ]

Additional context

I know that there are some supporting tiles in the screenshot I provided, but I tested this with a different building that burned down completely (all tiles on the ground floor) and got similar results. Also note how there are treetops but no trees underneath them.

[PatrikLundell]

/Confirmed

The save supplied doesn't seem to have any relation to the images posted. It loads with the PC standing on a roof, with no obvious burned buildings in the vicinity.

Edit: There are two reasons treetops remain:

• Treetops don't catch fire because the check to carry fire upwards requires a raging fire, and trees don't burn that hot (unlike bushes...).
• Terrain that produce ash just changes the terrain, unless other burning terrain, which is smashed, leading to whatever is above to be checked for lack of support.

The gutter issue is trickier, and probably can't be solved completely with the current logic for support (and the code has a comment the current implementation still has issues, but is better than it was earlier than that). It's partially an illusion, in that roofs of wooden buildings typically burn, and so disappear due to fire, rather than collapse, while gutters don't burn, and thus disappear only due to collapse. However, it seems the collapse_check leaves a bit to be desired, because the probability of collapse depends on support that may come solely from surrounding tiles that "support" each other mutually, while none of them actually is supported from below. There's also a bit of a cascading effect in that if one tile manages to stay intact when the supporting wall somewhere beneath disappears it will help the others to remain, while if the first one falls it's more likely further tiles will as well.

[Maarifrah]

Yes, I should have clarified that about the screenshots. The screenshots are from a separate save that I felt illustrated the bug more clearly. Instead of including that one, I wanted to provide a save that would include the process of the building burning down in the event that it was relevant.

To that effect, the save provided has a building to the right that is in the process of being burned down.